Raw pyrolysis gasoline is often hydrogenated in two stages, with intermediate fractionation so as to typically produce a C5 cut, various cuts intended to produce aromatic bases and gasoline bases or fuel oil. The existing process layouts generally allow to produce a C6 cut to extract benzene and a C7+ cut or a C6-C7-C8 cut to extract benzene, toluene and xylenes, and a C9+ cut.
By definition, a Cn cut is a cut essentially made up of hydrocarbons with n carbon atoms. A Cn+ cut is a cut essentially made up of hydrocarbons with at least n carbon atoms, up to hydrocarbons with 12 carbon atoms. This cut can generally comprise C13, or even C14 hydrocarbons. For example, a C8+ cut essentially comprises C8, C9, C10, C11, C12 hydrocarbons, and this cut can generally comprise C13, or even C14 hydrocarbons.
The C5 cut is generally recycled to the steam cracker or sent to the gasoline pool. The C6-C7-C8 cut, referred to as C6-C8 hereafter, essentially made up of hydrocarbons with 6, 7 or 8 carbon atoms, is used as the base for the production of aromatics (benzene, toluene and xylenes). The C9+ cut is generally used either as furnace fuel oil or as automotive gasoline base. In the latter case, it is generally necessary to separate the heavy fraction corresponding to an ASTM boiling point temperature above 220° C. from the C9-200° C. cut used as a gasoline base compatible with the gasoline cut points.
Besides, pyrolysis gasolines have high sulfur contents, notably the C9+ cut is often above the current (50 to 150 ppm weight) or future specifications. In fact, these gasolines contain about 300 ppm weight of sulfur, as well as high reactive unsaturated compound contents, which makes them unusable without an additional treatment.
The C6 or C6-C7 or C6-C8 fractions intended for the production of aromatic bases are treated in a dedienization stage (selective hydrogenation) in order to remove the reactive unsaturated compounds such as diolefins, acetylenic compounds and alkenyl aromatics, then in a hydrodesulfurization stage in order to remove the mono-olefins and the sulfur compounds, without however hydrogenating the aromatic compounds. Alkenyl aromatics are hydrocarbon-containing compounds consisting of at least one aromatic ring comprising at least one alkenyl group.
The C7+ or C8+ or C9+ fractions intended for the production of gasolines are often treated in a dedienization stage, then used directly as a gasoline base possibly after a fractionating stage to remove the C11+ or C12+ compounds and to obtain the end point specification for the gasoline. However, their sulfur content becomes incompatible with the evolution of the standards relative to the maximum sulfur content of gasolines that tend to fall below 50 ppm, or 30 ppm, or even 10 ppm weight.
Three options are currently used or considered for meeting this situation, in particular for existing steam crackers.
1) Option 1 consists in modifying the existing hydrotreating plants so as to significantly increase their capacity and desulfurization. There are suitable desulfurization catalysts, the most commonly used ones being mainly catalysts based on nickel and molybdenum, or nickel and tungsten or cobalt and molybdenum, on an alumina support.
2) Option 2 consists in adding a new final desulfurization plant for hydrogen treatment of the fraction valorizable to a gasoline cut.
These first two options lead to notable additional investments and hydrogen consumption, a gas that becomes increasingly rare on refining and petrochemistry sites, without any gain as regards valorization of the products that remain gasoline bases of rather poor quality. Furthermore, deep desulfurization is accompanied by a limited reduction in the aromatics content to be minimized, which remains unfavourable for the octane number and therefore for its valorization.
3) Option 3 consists in selling the gasoline fraction as produced to an oil refinery that will achieve final desulfurization. This option leads to a significant depreciation of the price of the gasoline thus sold.
The goal of the invention is to find a technically simple and inexpensive solution to the aforementioned problem in order to produce on a petrochemical site C7+ or C8+ or C9+ fractions from steam cracking plants that can be directly used as gasoline base with a low sulfur content.
The various ways of hydrotreating liquid hydrocarbon fractions from steam cracking plants are described in the literature. An example thereof is patent application FR-2,858,981 that describes a production layout for various cuts from a steam cracking plant using 3 distinct hydrotreating stages.
However, the existing solutions or those considered exclusively consist in carrying out hydrodesulfurization stages that require the presence of hydrogen in a costly process and they do not describe the possibility of treating one of the fractions from the steam cracking plant by means of a process based on weighting the sulfur compounds on an acid catalyst.
Besides, desulfurization of hydrocarbon fractions by treatment on an acid catalyst is also widely described in the literature. For example, U.S. Pat. No. 6,048,451 describes how to desulfurize gasolines from catalytic cracking plants by means of a process consisting in converting the sulfur compounds to heavier sulfur compounds using an alkylation agent in the presence of an acid catalyst. The alkylation agent includes olefins or alcohols. However, this invention is described for an application to catalytic cracking gasolines and it aims to weight sulfur compounds of thiophene and methylthiophene type.